Frequency modulator



` Patented Sept. 18, 1945 FREQUENCY MODULATOB David Arthur Bell, London, England, assignor to Badiolatents Corporation, a corporation of New York Application February 11, 1942, serial No. 430.385 In Great Britain February 13, 1941 s claims. (ci. 17a-171.5) 4 l 'I'his invention relates to the generation of electrical oscillations.

One feature of the invention is the method of generating a frequency modulated oscillation winch comprises modulating the relative amplitudes of two different constant frequency oscillations which are added and applied to a non-linear conductor.

When, for example, two alternating voltages of different frequencies are added and applied to a limiter, one component oi the output voltage from the limiter has a frequency between the frequencies of the applied alternating voltages and its frequency will depend on the relative amplitudes of the two alternating voltages. If, therefore, one of the alternating voltages be modulated in amplitude, or if both be modulated in amplitude in opposite phase, the frequency of this output component will be correspondingly modulated.

If the amplitude of one of the alternating voltages is kept constant, while the amplitude of the other is varied' within the limits 1.0 to 0.4 times that amplitude, then the frequency varies substantially linearly with the amplitude ratio. The

corresponding range of frequency' variation is from the mean of the frequencies of thevalternating voltages to a departure from that mean of about 36% of the difference between those frequencies.

If the amplitude of one of the alternating voltages is increased while the other is decreased by an equal amount, the variation of frequency with the 'amplitudes is substantially linear so long as the ratio of either amplitude to the mean amplitude lies between the limits 0.8 and 1.2. 'I'he corresponding frequency variation to each side of the mean frequency is about :19% of the difference between the frequencies of the alternating voltages.

Over the greater part oi' these ranges the limiter wili maintain substantially constant amplitude of the desired component of the output volt-` age. When, however, the amplitudes of the applied alternating voltages are equal.- the amplitude of this component falls to zero. If it is desired to maintain its amplitude at frequencies in the immediate region of the mean frequency, this may be effected by applying to the-limiter an additional input alternating voltage having the mean l frequency. This will, however. have the effect of reducing the range of frequency modulation obtained for a given variationfof amplitude ratio of the input alternating voltages.

If it be desired to obtain frequency modulation over wider ranges than those above mentioned,

without loss of linearity, pre-distortion of thel modulating voltage may be employed.

A further feature of the invention is directed to the problem of separating components of sum and difference frequencies, which are generated by mixing oscillations of constant amplitude and different frequencies. In the arrangement according to this feature of the invention two mixers are employed and alternating voltages of both input frequencies are applied to each mixer. Those applied to one mixer, however, are in phase quadrature with those applied to the other mixer. By adding in equal amplitude but in-opposition the output currents of the two mixers, a current having a component of the sum frequency is obtained substantially free from any component of the difference frequency. By adding them cumulatively, on the other hand, a component of the difference frequency is obtained substantially free from any component of sum frequency.

This feature ofthe invention is particularly useful when it is desired to separate sum and difference frequencies which are so close together that it is difilcult to separate them by means of filters.

Substantial components having the frequencies of each of the input alternating voltages will normally appear, together with some of their harmonics, superimposed on each of the separated sum and difference frequency components.- If the sum and diierence frequencies are close together, the component having the lower input frequency, and at least the more substantial harmonics, can easily be separated by filters; but this is not true of the component having the higher input frequency. By using for the mixers either balanced modulators or mixers of substantially square law. type, however, the components having the input frequencies can be eliminated.

A further feature of the invention is the method of generating a frequency modulated electrical oscillation which `comprises mixing oscillations from two sources, o ne of approximately carrier i ment ofthe carrier frequency and of the hand width can be achieved.

In the accompanying drawing. Figure 1 is a circuit diagram of a frequency modulation generator; Figure 2 is a circuit diagram of a generator f of two alternating voltages having frequencies equal to the sum and difference of the frequencies of two alternating sources; and Figure 3 is a schematic circuit diagram of a frequency modulation generator embodying the main principles of operationof the generators shown in both Figures l1 and 2.

In the generator of which Figure 1 is a circuit diagram, the tuned circuits l and 2 are resonant at frequencies respectively above and below the range of frequencies of the frequency modulated voltage to be generated.

The triode oscillator section of the triode hexode valve 3 is connected to form with tuned circuit a conventional Hartley oscillator. The output of this oscillator is applied in the usual way to the outer control grid of the hexode modulator section of valve 3, and the modulating voltage is applied from kmodulation source I to the inner control grid. Bias for b'oth control grids is obtained from resistor 5 provided with by-pass condenser 6. The screen grid is connected to the high tension source, and is maintained at cathode potential with respect to alternating currentsby condenser 1.

The oscillator associated with tuned circuit 2 is not required to be modulated. A conventional electron coupled oscillator comprising pentode valve 8 is shown. y

The anode currents of valve 8 and of the hexode section of valve 3 are passed in parallel through output load impedance 9, which is designed to resonate at the frequencies of the oscillations generated in valves 3 and 8 and over the whole frequency range between them.

'I'he mean amplitude of the oscillation current from valve 3 in impedance 8 is adjusted to, say, about 0.6 times the amplitude of that from valve 8. The modulation range of the oscillation current from valve 3 is from 0.4 to 0.8 times that amplitude. l tained which is substantially linear with oscillation current from 4valve 3. and which has a range of about of the difference between the frequencies of the oscillations generated in valves 3 and 8.

The alternating voltages developed across load impedance 9 are applied to limiter lll and the output therefrom is applied to an output impedance I|, which is designed to resonate over the whole frequency range of the frequency modulated voltage to be generated, but not to resonate at the frequencies of the oscillations generated in valves 3 and 8.* The output voltage of the generator is developed across impedance l and appears at output terminals l2.

The'function and operation of the invention will be further -understood from the following. Let two sinusoidal oscillations A sin wit and B sin wat be represented by two vectors of constant angular velocities, w1 and wz, respectively. If .r is the length at any instant of the resultant vector representing the sum of these vectors and o' is the angle it makes with the reference direction at any time t, then:

T=JA2+B2+2AB cos (w1-wz): (I)

p and 4 115mm tan A cos wlt-i-B cos wzt (n) A variation of frequency is then ob-f Substituting u=A sin wit-HB sin wat and v= A cos wim-Boos wat in Equation II A sin wxt-i-B sin wat n i n..

di dt a A cos wlt-l-B cos wat The terms involving cos (wr-wn) may he nebeyond the range of modulating frequencies. Accordingly, the instantaneous angular velocity may be represented as follows:

dz- .i1-rill From (IV) it follows that de dt or the instantaneous frequency will lie between w1 and un but will vary according tothe relative magnitudes of A and B and for any given values of A and B,

'the two component frequencies. In order to extractthe oscillation of frequency it is necessary to perform on the actual signal a physical operation corresponding to the mathematical operation of discarding the amplitude and considering only l the frequency. This physical operation consists in passing the signal through a limiter.

In the generator illustrated in Figure 2, the alternating voltage of constant amplitude derived from source 20 is applied directly to the inner control grid of hexode mixer valve 2| and through quadrature .phase shifting device 22 at the same amplitude, to theinner control grid of hexode valve 23. The alternating voltage of constant amplltude derived from source 24', of different frequency from source 20, is applied directly to the outer control grid of hexode mixer valve 23 and through quadrature phase shifting device 25', at

the same amplitude, to the outer control grid of gastarse4 form ,separate output impedancesin the anode circuits of valves 2i, 23. The tuned circuit con-y y frequency equal to the sum o f the frequencies of sources and 24 is developed between terminals 34, this voltage representing the resultant obtained by adding Ain opposition the voltages developed across the two halves of tuned circuit 30, 3|. This voltage is balanced about earth. The output voltage, developed between terminals has frequency equal to the difference between the frequencies of sources 20 and 24; this output voltage is asymmetric.

In the generator represented by the schematic circuitv diagram Figure 3, source supplies an alternating voltage having the carrier. frequency of the frequency modulated voltage oscillation to be generated. Source 4I supplies an alternating voltage having a constant freueqncy a few times greater than the range of frequency modulation required. The oscillations of constant amplitude and of different constant frequencies derived from sources 40 and 4| are applied directly to mixer 42 and through quadrature phase shifting devices 43, 44 to mixer 45. The separate output circuits of mixers 42 and 45 comprise im.. pedances 46, 41 respectively, which offer high impedance at the frequency equal to the sum of the frequencies of sources 40 and 4I and low impedance at harmonics of these frequencies. The common output circuit of mixers 42 and 45 coinprises impedance 48 which offers high impedance at the frequency equal tothe difference of the frequencies of sources 40 and 4I and low impedance at harmonics of these frequencies.

The output voltage 'developed across impedance 48 is asymmetric and is` applied directly to excite amplitude modulator I9. The output voltage obtained by combining in opposition the output voltages developed across impedances 46 and 41 is symmetric. Transformer 50 indicates diagrammatically means to convert this voltage to asymmetry before 'application to excite amplitude modulator 49. Modulating voltage derived from modulating source 5I, which may be asym. metric, is passed through a device represented by transformer 52 by which it is rendered symmetric before application to modulators 49 and 59. The output currents of modulators 49 and 58, amplitude modulated in opposite phase, are passed through a common output impedance 53 having resonant band width including the sum and difference frequencies of sources 40 and 4| and covering the whole range between them. The voltage developed across the impedance 53 is applied to limiter 54 and the output voltage thereof is applied to final output impedance 55, the resonant band width of which includes all frequencies within the range of frequency modulation required but excludes the sum and differthrough the modulators 49 and 59, this further voltage which is derived direct is preferably applied in quadrature with saidcomponent. This will occur without the provision of a phaseshifter in lead 56.

'I'he carrier frequency is determined by source 40 alone, and its constancy canV therefore be maintained by control of a single oscillator. The band-width is controlledl by the frequency of source, 4I. i

In some cases the modulators may themselves serve as limiters, in which event output impedance 53 and limiter 54 may be omitted from the circuit. Carrier frequency derived direct from source 40 may be injected, if required, in final output impedance 55. A degree of limitation is obtained in the modulators when hexodes are employed, and their anode potentials are swinging below their screen grid potentials, with consequent appearance of low internal resistance. This limitation affects the total voltage at the anodes, but does not control the proper-'- tion of the voltage due to each of the two valves; it'is therefore possible for the amplitude-modu- 5 lation ofsthe two valves separately to vary the composition and hence the effective frequency of the output voltage, in spite ofthe tendency for the total voltage to be kept constant. The limiting action should therefore be regarded as occurring in the commonoutput circuit.

While the foregoing description applies to a. singlemodulating frequency in which `case the circuit which fundamentally produces a phase modulated oscillation likewise generates a frequency modulated wave, a frequency modulated output wave may be obtained in case of a more `complex modulating signal comprising more than a single component frequency such as speech or music by the employment of an integrating corrective network in the modulating circuit adapted to render the amplitude of themodulating signal components inversely proportional to their frequency in a manner well known in the art.

1. A generator of a frequency modulated voltage oscillation of substantially constant amplitude, comprising a pair of amplitude modulators, means to excite said modulators with oscillations of two different frequencies outside the range of the frequencylmodul'ation, means to apply modulating voltages from a common modulating source to both of said modulators to modulate said oscillations in opposite phase, a common output impedance for said modulators, having a resonant band width including both of the frequencies of said oscillations and the whole range between them, a voltage limiter deriving its input voltage across said modulator output impedance, and a final output impedance receiving the output of the limiter and having a resonant band width including all frequencies within the range of frequency modulation but excluding the frequencies of said oscillations.

2. The method of generating a frequency modu- 5 lated electrical oscillation which comprises mixing oscillations from two sources, one of approxilmately carrier frequency and the other of a frequency which determines the ratio of frequencyV modulation band width to amplitude of the modulation source, separating oscillations having the sum and difference frequencies of the sources, modulating the relative amplitudes of the oscillations of sum and difference frequencies and combining them, and limiting the amplitude of the 5 combined waves to extract an electrical oscillation having its frequency modulated in accordance with the amplitude of the modulation source.

3. A frequency modulated oscillation generator comprising a pair of sources of oscillations one of approximately carrier frequency and the other having a frequency determined by the frequency modulation band to be obtained, means for intermodulating the oscillations of both said sources to produce sum and difference frequency energies, further means for modulating the relative amplitude of the derived sum and difference frequency energies in accordance with a modulating signaLmeans for combining the modulated sum and difference frequency energies, and limiting means for removing amplitude modulation from the combined energy.

4. A frequency modulated oscillation generator comprising a pair of sources of fixed frequency oscillations, one of approximately carrier frequency and the other having a frequency determined by the frequency modulation band to be obtained, means for intermodulating the oscillations of both said sources to produce sum and difference frequency energies, further means for modulating the produced sum and difference frequency energies in opposite phase in accordance with a modulating signal, means for combining the modulated sum and difference frequency energies, and lim'- iting means for removing amplitude modulation from the combined energy.

5. A frequency modulated oscillation generator comprising a pair of sources of fixed frequency oscillations, one of approximately carrier frequency and the other having a frequency determined by the frequency modulation band to be obtained, a pair of mixers, means to apply input voltages of both source frequencies to each mixer, each of the input voltages to one mixer being in quadrature with the input voltage of the same frequency to the other mixer, separate output circuits for said mixers and a common output circuit of both mixers, means to derive sum and dierence frequency energies from said separate and said common output circuits, respectively, means for modulating the relative amplitude of the derived sum and difference frequency energies in accordance with a modulating signal, means for combining the modulated sum and difference frequency energies, and limiting means for removing amplitude modulation from the combined energy.

asoman 6. A frequency modulated oscillation generator comprising a pair of sources, of fixed frequency oscillations, one of approximately carrier frequency and the other having a frequency deter` mined by the frequency modulation bandto be obtained, a pair of mixers, means to apply input voltages of both source frequencies to each mixer, each of the input voltages to one mixer being in quadrature with the input voltage of the same frequency to the other mixer, separate output circuits for said mixers and a common output circuit of both mixers, means to derive sum and difference frequency energy from said separate and said common output circuits, respectively, further means to modulate the derived sum and difference frequency energies in opposite phase in accordance with a modulating signal, means for combining the modulated energies, and limiting means for removing amplitude modulation from the combined energy.

7. A frequency modulated oscillation generator comprising a pair of independent oscillation sources having substantially constant frequencies located outside the range of the desired frequency modulation band, means to modulate the relative amplitude of the source oscillations, means for combining the modulated oscillations, and limiting means' for removing amplitude modulation from the combined oscillation energy.

8. A frequency modulated oscillation generator comprising a pair of amplitude modulators, means to excite said modulators with oscillations of two different frequencies outside the desired range of frequency modulation, means to apply modulating voltages from a common modulating source to both said modulators to modulate said oscillations in opposite phase, the ratio of either amplitude of said oscillations to the mean amplitude remaining within the limits .8 to 1.2, a common output impedance for said modulators having a frequency response band including both of the frequencies of said oscillations and the whole range between them, a limiter arranged to derive its input voltage from said output impedance, and a final output impedance receiving the output of said limiter and having a frequency reponse band including all frequencies within the range of frequency modulation but excluding the frequencies of said oscillations.

DAVID ARTHUR BELL. 

